Calcia-Magnesia-Alumino Silicate (CMAS) is a form of molten siliceous residue generated at elevated temperatures within aeroengines. CMAS adheres to the surface of thermal barrier coatings (TBCs) and has the potential to cause significant damage to engine components, resulting in TBC failures. The aviation industry has long recognized CMAS as a substantial threat to aircraft engines, and this threat persists today. A substantial amount of research has been carried out, primarily focusing on gaining a fundamental understanding of the degradation mechanism of traditional TBCs manufactured using air plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD) technologies after CMAS attack. A thorough understanding of why CMAS forms, its role in causing severe spallation, and how to prevent it is of significant concern both academically and industrially. This review article provides a detailed examination of the chemistry of CMAS and the resulting degradation mechanisms that the TBC may encounter throughout the aeroengine service life. This article also explores recent research, incorporating case studies, on the impact of CMAS attack on the resulting chemical and structural modifications of the ceramic topcoats. Current strategies designed to mitigate CMAS infiltration and perspectives for enhanced mitigation are discussed.

Thermal spray deposition techniques have been well-established, owing to their flexibility in addressing degradation due to wear and corrosion issues faced due to extreme environmental conditions. With the adoption of these techniques, a broad spectrum of industries is experiencing continuous improvement in resolving these issues. To increase industrial-level implementation, state-of-the-art advanced materials are required. High-entropy alloys (HEAs) have recently gained considerable attention within the scientific community as advanced materials, mainly due to their exceptional properties and desirable microstructural features. Unlike traditional material systems, high-entropy alloys are composed of multi-component elements (at least five elements) with equimolar or nearly equimolar concentrations. This allows for a stable microstructure that is associated with high configurational entropy. This review article provides a critical assessment of different strengthening mechanisms observed in various high-entropy alloys developed by means of deposition techniques. The wear, corrosion, and oxidation responses of these alloys are reviewed in detail and correlated to microstructural and mechanical properties and behavior. In addition, the review focused on material design principles for developing next-generation HEAs that can significantly benefit the aerospace, marine, oil and gas, nuclear sector, etc. Despite having shown exceptional mechanical properties, the article describes the need to further evaluate the tribological behavior of these HEAs in order to show proof-of-concept perspectives for several industrial applications in extreme environments.

Surface phenomenon such as cavitation erosion-corrosion limits the working life and durability of the fluid machines through significantly altering the efficiency. Surface modification is an apparent and economical route for improving the sustainability of these components. Recently developed complex concentrated alloys (CCAs) or high entropy alloys (HEAs) possess exceptional properties owing to high configurational entropy. We developed CCA coatings on the stainless steel using a facial and effective microwave processing technique. The effect of Al molar fraction in AlxCoCrFeNi (x = 0.1-3) CCAs on ultrasonic cavitation erosion-corrosion was investigated in 3.5% NaCl solution. For comparison, cavitation erosion and electrochemical corrosion behavior of the pre- and post-tested samples was also performed. Detailed microstructure and mechanical characterization of the developed coatings were also preformed using different analytical techniques. The equimolar CCA coating showed apical degradation resistance under both pure erosion and erosion-corrosion conditions. The observed behavior is attributed to high strain hardening, optimal hardness, fracture toughness, and utmost stability of the passive layer. The phenomenal conjugation of these properties was associated with highest configurational entropy for equimolar composition resulting in sluggish diffusion, and severe lattice straining. Compared to pits, striations and cracks characterizing the morphology of the degraded stainless steel, the equimolar and Al0.1CoCrFeNi CCAs showed TTS (tearing topograph surface) as the dominant failure mode characterized by presence of microplastic deformation. The degradation of the Al3CoCrFeNi CCA occurred mainly through brittle failure mode. The difference in failure mechanism is related to the mechanical properties and underlying microstructure.